When TV stars date their co-stars, it feels like a match made in fanfic heaven. But all that pressure has the potential to rock the boat...or ship, as it were. Whether it’s leads to self-referential humor, an early character departure, or a surprise pregnancy, a behind-the-scene romance has the power to change a plotline. And likewise, an on-screen relationship has the power to shake a real one. See for yourself below.

1. Adam Brody and Rachel Bilson

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The O.C. was always golden with meta humor, and no example of that is greater than “The L.A.” Centered around show-within-a-show-about-the-show, The Valley, Summer spends time with Grady Bridges, a clear Adam Brody stand-in. Grady comes on to her just before April, clearly meant to be that world’s Rachel Bilson, walks in and flips out.

“Wait, you two are dating,” Summer says. “God, don't you think that's a bad idea. I mean, what if things don't work out and you guys break up. I mean, isn't that bad for the show?”

Lolz aplenty, because at that point in the show Adam and Rachel were dating. And though it only bled into the show through parody, it was a bad idea.

“I just had absolutely no idea that all that – being on The O.C., dating Adam [Brody] – was going to end up being so invasive,” Rachel later told Lucky Mag. “I don’t mind being the center of attention as a character but in real life it’s not for me.”

2. Sophia Bush and Chad Michael Murray

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When Sophia Bush and Chad Michael Murray ended their five-month marriage, it may have taken One Tree Hill's Brucas down with it. Hell, it may have taken Chad Michael Murray down with it. The show initially centered around the love triangle between Peyton, Brooke, and Lucas, but Brooke took herself out of the running in season four after a pregnancy scare made her re-evaluate their relationship. Aaaaand Peyton swept in for the win, ultimately marrying Lucas and having his baby in season 6.

Hearsay is that Sophia and Chad were able to keep things professional through the next two years, but when CMM bounced in season 6, it wasn’t surprising. Still, without that real-life marriage, would Chad have renewed his contract? Would Sophia and Chad had saved Brooke and Lucas? Guess we’ll never know.

3. Blake Lively and Penn Badgley

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For a time, the cast of Gossip Girl mirrored the quasi-incestuous relationships between the characters of Gossip Girl. Granted, the unions of Leighton Meester + Sebastian Stan and Jessica Szohr + Ed Westwick only resulted in brief flings. Nothing was going to stop the Chair train. But Penn Badgley and Blake Lively’s romance was unique: the relationship was urged by the producers, who thought it would help fuel Dan and Serena’s storyline.

"I remember there was one point where we were just afraid of how our personal lives overlapping our work life could be perceived by our bosses," Blake told Vanity Fair. "[But then] we were like, ‘Oh no, that’s exactly what they want.’ They wanted us all to date. They wanted us all to wear the same clothes that we’re wearing on the show. They wanted that, because then it fed their whole narrative. People could buy into this world."

Of course, Penn and Blake eventually broke up, making it super awks when (spoiler) Derena ended up married and Dan ended up being effing Gossip Girl.

Fun Factoid, though: Blake’s real life relationship with Leonardo DiCaprio also served as inspiration for the show, motivating the writers to move the series to L.A. for a few episodes.

4. Emily Vancamp and Dave Annable

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Brothers & Sisters was meant to be a show about, well, brothers and sisters, but something tricky happened when long lost half-siblings Justin and Rebecca met. Due to the attraction between Dave Annable and Emily Vancamp, their chemistry crackled like... well, lovers.

”I looked like an idiot,” executive producer Greg Berlanti recalled at the time. ”Here were two young people who didn’t grow up with each other, didn’t know each other, but they had this electricity. There was nothing I could do.”

Actually, there was something they ended up doing: they created a paternity scandal to redact Rebecca’s parentage and make it so the two were very not-related. It worked: Justin and Rebecca end up together, though Dave and Emily have since parted ways.

5. Joshua Jackson and Katie Holmes

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There’s a reason why the Joey and Pacey relationship felt so real: Joshua Jackson and Katie Holmes were involved throughout the first two seasons of Dawson's Creek, and Katie alluded to him being her “first love.” The crew saw that chemistry early on — and decided to do something about it.

“In season 1, Joey and Pacey are rained on while outside collecting samples for a science project,”series creator Kevin Williamson shared. “They got wet and had to take their clothes off in the truck and there was that very uncomfortable moment where you could just see how she peeked at him and he peeked at her. They were so nervous with each other. They had instant chemistry.”

And the rest is history."We all saw that and we were like, 'OK, uh, [do we pair them up in] season 3,” he continued. “Season 2?!"

6. Jennifer Aniston and Tate Donovan

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A tip to the hat to Jennifer Aniston and Tate Donovan. You may know they dated in the ‘90s, and you may know their characters dated on Friends, but the beginning of a story arc was actually an ending for the real life couple.

Through set visits to his one-time fiancée, Tate got familiar with the cast and crew, so it wasn’t long before the creators offered him a guest spot. Unfortunately, the timing was off: Tate and Jen were splitting up when the offer came, and though she didn’t stop him from taking the part, it was an excruciating experience for all involved.

“It was horrible,” Tate shared in a 2013 HuffPost Live interview. “So tough. It was so tough, man. I remember just getting back to my dressing room and just weeping.”

I get it, man. Pretending to meet and fall in love with your girlfriend must be rough when in reality your relationship is unraveling.

7. Jennifer Aniston and Brad Pitt

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And while we’re at it, let’s just talk about how Brad Pitt nearly drove a wedge between America’s most famous on-again, off-again couple. Brad Pitt lowered himself to the realm of sitcomdom because of his relationship with, you guessed, Jennifer Aniston. Also, according to Wikipedia, to promote Spy Games, but whatever.

In the Friends episode, Brad played Will, a former classmate of Ross, Monica, and Rachel and former president of the “I Hate Rachel Club.” Over the course of a complex-carb rich Thanksgiving dinner, Will reveals that he and Ross once started a vicious rumor about Rachel (I’m not repeating it here, suffice to say it didn’t age well). She’s momentarily livid at Ross and orders him to call their former classmates and redact the rumor.

At this point in the series, Ross has already knocked up Rachel. Though they’re not together — a fact Will takes delight in — they’re still pretty much endgame. Nothing is going to stop those two from having their happily ever after.

The same could not be said for Brad and Jen, but you probably knew that already.

8. Nina Dobrev and Ian Somerhadler

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It’s not crazy to think that Nina Dobrev and Ian Somerhadler pushed The Vampire Diaries' Elena and Damon together ahead of schedule. It’s also not crazy to think that their 2013 breakup pushed Nina to leave the show ahead of it wrapping up, riddling the series with a bunch of plot holes. Plenty of fans consider that rumor to be just ugly tabloid gossip, and maybe there’s something to that: Nina and Ian have remained friends, and Damon and Elena still ended up endgame.

Interestingly enough, though, Stelena could’ve pulled through if Nina hadn’t left the show. “Well, I always thought it would be Stefan and Elena,” co-creator Kevin Williamson told Deadline. “They were sort of the anchor of the show, but because we lost Elena in season 6, we couldn’t go back. You know Nina could only come back for one episode — maybe if she had came back for the whole season, we could even have warped back towards that, but you can’t just do it in 42 minutes.”

Oh, the irony!

9. The Entire Cast of 'Saved By the Bell'

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Thanks to that whole unauthorized Lifetime movie, we all know about Mark-Paul Gosselaar and Lark Vorhies having a relationship. But in true Zack Morris fashion, MPG also dated Tiffani Thiessen and Elizabeth Berkley.

Likewise, Tiffani couldn’t choose between Team Zack and Team Slater: she dated Mark-Paul and then Mario Lopez. In his book, Dustin Diamond alleges that Tiffani was with both at the same time, all the while cheating on boyfriend Eddie Garcia. I remain suspicious of that rumor, but fun thing about Eddie Garcia: he ended up playing pot-smoking actor Johnny Dakota on the show.

“All of us dated at one point or another—it was incestuous!” said Mark-Paul in a 2009 spread with People.“Sometimes the girls would gang up on the guys. Tiffani and Elizabeth would hate me, and then they’d hate Lark because Lark was talking to me, and Mario was supposed to side with someone.”

“All that stuff you did in high school, like, ‘How could you talk to him,'” Tiffani-Amber Thiessen added. “Did we have crushes on our costars? Absolutely. But we were so young!”

10. Ginnifer Goodwin and Josh Dallas

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Josh Dallas and Ginnifer Goodwin really have a fairy tale romance. They met on the set of Once Upon a Time and got to have a real Prince Charming and Snow White romance... while playing Prince Charming and Snow White.

“I’ve learned and learned and learned as an actress and teammate,” mused Ginnifer at the time. “And, I mean, look at my life! Over the run of this show, I met Prince Charming, fell in love, got married, and had two babies. It doesn’t get better than that.”

Or more Disney than that. But about those two babies: when Ginnifer got pregnant with her first child, Snow White followed suit! The OUAT writers were happy to meld the pregnancy plot into the storyline, especially since it was actually Josh’s child and that could only mean solid emotional gold. That said, the second time around they decided to work around Ginnifer’s baby bump.

11. Bonus: Lara Flynn Boyle and Kyle Maclachlan

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The first season of Twin Peaks teased a heavy attraction between Audrey Horne and Agent Dale Cooper, but his gentlemanliness had him shrugging off her advances. Or at least, that was the excuse. Although Sherilyn Fenn and Kyle MacLachlan kept things strictly professional, the Special Agent had an off-screen romance with another castmate: Lara Flynn Boyle. And well, let’s just say Donna was the iceberg for the Audrey and Agent Cooper ship.

“So [MacLachlan’s] girlfriend, Lara Flynn Boyle, kiboshes an astonishing thing,” Sherilyn shared on the Twin Peaks Unwrapped podcast. I remember saying, ‘David, is this how it goes? An actor complains, because she’s the girlfriend, and then you change?’”

Yup. It was written that Dale couldn’t pursue Audrey because she was “too young” (Sherilyn was in her twenties, but playing an 18-year-old). “Literally, because of that, they brought in Heather Graham — who’s younger than I am — for him and Billy Zane for me,” Sherilyn continued. “I was not happy about it. It was stupid.”